Manufacturers and designers of performance footwear, which require tight and compressive lacing, strapping, buckling or attachment with a binding or other pressure or friction system of attachment, struggle to build footwear that fits well, is comfortable, provides consistent fit and is flexible enough to allow the manufactured unit to fit the many variations in sizes and shapes of feet. Manufacturers generally use layers of foams and padding materials to compensate for these natural variations, to promote comfort and protection and to universally contour the interior of the footwear to the wearer's feet. In addition to comfort and fit, the footwear must also provide support and control.
It is known that the anatomical structure of the foot causes the weight of a person to be supported at the heel and the head of the first and fifth metatarsal bones. The magnitude of forces associated with side-to-side and lateral motions, combined with the impact and shock generated by sudden stops and starts, is great in performance footwear. An athlete needs to transfer energy and weight from his or her upper body, and through the legs, ankles and feet, to generate enough energy to propel the athlete and, yet, maintain a relaxed and comfortable position to absorb the resulting shock and impact. Performance footwear is typically designed with laces, straps, binds and other pressure or friction systems of attachment in an attempt to ensure control and perfect contact between the footwear, the user's foot and any sport appliance, such as the blade of a skate, a ski or a snowboard. Other footwear, such as basketball shoes or climbing boots, includes a shaft or collar which extends over the ankles to provide support and control.
However, although the above described measures provide some support, they fail to adequately address the effect of the strong compressive forces transmitted to the instep are of the foot caused by the very lacing, strapping, buckling or other attachments used to provide the support. The measures also do not adequately or consistently maintain the user's foot in position within the footwear or boot, and more specifically, do not maintain the user's heel in a fixed and comfortable position.
At present, foam pads are generally the primary means utilized by manufacturers of supporting and cushioning apparel and other protective gear to reduce injury. However, foam merely flattens directly under the point of pressure and does not redirect the pressure or energy of the impact. Although foam acts as a shock absorber, it is incapable of acting as an energy absorber. For example, currently, the accepted remedy for "bone-out" or protruding first metatarsal bone is to use various types of foam to cushion the area. Yet, foam does not flow or conform to specific shapes. Foam merely compresses and flattens under an external load. Using foam as a cushioning material and to merely cover tender spots results in restricted circulation and does not reduce discomfort and bruising.
Shock absorbing materials such as foam compress so quickly under pressure that they are unable to absorb enough energy to significantly reduce impact trauma. Thixotropic liquids such as those described in U.S. Pat. No. 5,869,164 to Nickerson, which is a mixture of microspheres in oil and a thickener, are heavy in weight and, because they are comprise a liquid medium, they are non-compressible and therefore behave like a supporting device and do not reduce trauma or provide impact protection. As such, although shock absorbers reduce the risk of surface injury, they do not significantly reduce injury to the underlying tissues because a substantial portion of the energy is transferred to the underlying tissues. In addition, such liquid based devices are subject to puncture, susceptible to leaking and are complicated to manufacture because of their complex formulations.
In addition, although it is described in U.S. Pat. No. 5,869,164 that glass and plastic microspheres may be mixed with thixotropic liquids, the microspheres are merely suspended in the thixotropic liquid and thus are free to move around within the liquid. This freedom of movement allows the microspheres to be pushed to, and concentrated in, areas of the thixotropic liquids which are not subjected to pressure. Movement of the microspheres thus reduces the effectiveness, especially over extended periods of use, of thixotropic liquids.
Moreover, bonding agents, such as polyisobutylene polymers, which are typically used in such cushioning devices, are almost always non-liquid at room temperature because of the molecular weight, chemical composition and thermoplasticity. As such, before working with these polymers and to make them flowable, the temperature of these polymers must be raised to lower their viscosity.
Resilient, conforming materials comprising microspheres are also described in U.S. Pat. No. 4,252,910 to Schaefer. Specifically, Schaefer describes a material in which gas-filled microspheres are cohered to a mass by a bonding agent; wherein Schaefer's microspheres consist of an elastic copolymer preferably of vinylidene chloride and/or vinyl chloride copolymerized with acrylonitrile. However, the formulations of Schaefer have such a high viscosity and sticky nature make the resulting materials virtually impossible to handle and are useless for most applications. For example, Schaefer's material is non-liquid at room temperature and, according to Schaefer, the user must warm his or her foot above normal body temperature to soften Schaefer's material enough to take the shape of the user's foot. Moreover, Schaefer teaches that his material must be at least at body temperature to be flowable. In addition, Schaefer's material has a very high ratio of polymeric material to microspheres, namely, about 53:1. Furthermore, Schaefer is unable to substantially increase the number of microspheres per unit volume because of the high viscosity of Schaefer's material. The low number of microspheres in Schaefer's material severely limits the number of interstices per unit volume which, in turn, reduces the dilatent strength of Schaefer's material.
A further disadvantage of polyamide and polyisobutylene synthetic polymers as a binding agent is that the resulting cohered mass of microspheres shows a high degree of compression set (low compression regain) which limits the mass' usefulness. This is especially true when such materials are used in cushioning applications.